This application relates to semiconductor wafer manufacturing.
Wafers of semiconductor material can be formed by slicing or cutting pieces from a semiconductor ingot. Cutting devices such as internal diameter (ID) diamond saws or abrasive wires are used to slice the wafers from the ingots.
One wafer fabrication technique involves securing an ingot to a holding strip, usually with an adhesive material, and plunging a saw blade through the ingot and partially through the holding strip. The saw blade retracts without severing the slice from the rest of the holding strip. Leaving the holding strip intact in this manner prevents the newly formed wafer from falling into the saw blade housing or the saw""s fluid catch pan. This technique requires manual or mechanical separation of each slice, including both the wafer and the portion of the holding strip to which the wafer is connected, from the rest of the holding strip.
FIG. 1 shows an ingot 100 resting in a conventional holding strip 102. The holding strip 102 is generally rectangular in cross section, with a groove or trench 104 formed in one surface to accommodate the ingot 100 during the wafer cutting process. In general, the holding strip 102 is formed from a material, such as graphite or aluminum oxide, that is much softer than the semiconductor ingot 100 itself. As the cutting edge 106 of the saw blade or other cutting device passes through the ingot 100 and penetrates the holding strip 102, the softer material in the holding strip 102 causes vibration and deflection of the saw blade. This vibration and deflection often causes the blade to chip the edges of the ingot 100 and the newly-formed wafer, damaging the ingot and wafer surfaces and reducing the yield of useful wafers. Reduced yield leads to higher labor and material costs, which in turn lead to higher prices at the consumer level.
Holding strips that are softer or harder than the semiconductor ingots also cause premature dulling of the saw blade and formation of a powder layer on the blade. These conditions reduce the cutting efficiency of the saw blade and lead to more frequent reconditioning or disposal of the saw blade.
Moreover, the rectangular cross section of the holding strip 102 gives the strip a relatively high breaking strength. High breaking strength makes it more difficult to separate the slices from the rest of the holding strip 102 and therefore adds to the cost of the wafer production.
This application provides techniques for reducing chipping of semiconductor wafers during the cutting process and for reducing the breaking strength of partially cut holding strips. These techniques lead to higher wafer yield and reduced wear-and-tear on wafer cutting devices. As a result, the costs associated with wafer fabrication, and thus the ultimate costs of consumer goods, are lower when these techniques are used during wafer fabrication.
The invention is useful in the production of semiconductor wafers from a semiconductor ingot. In some aspects, the ingot rests against a holding strip that is formed from a semiconductor material, typically the same material used to form the ingot. A wide variety of semiconductor materials, including single-crystalline and polycrystalline materials, can be used to form the holding strip.
In other aspects, the holding strip has a holding surface shaped to receive the ingot and at least one surface other than the holding surface, into which at least one notch is formed. The holding strip has a characteristic breaking strength that changes when a cut is formed through the holding surface and into the notch. In some embodiments, the notch has sides that are substantially parallel to each other, and in other embodiments, the notch has tapered sides. In alternative embodiments, the shape of the notch causes an abrupt change or a gradual change in the breaking strength of the holding strip as the cut penetrates into the notch. In either case, the notch can be shaped to cause a gradual change in breaking strength as the cut moves deeper into the notch.